Resistor with a tubular terminal



R. L. JONES Dec. 5, 1967 RESISTOR WITH A TUBULAR TERMINAL Filed June so,1965 5 7 E mw M L W E w R United States Patent 3,356,981 RESISTOR WITH ATUBULAR TERMINAL Robert L. Jones, Riverside, Califl, assignor to BournsInc., a corporation of California Filed June 30, 1965, Ser. No. 468,40211 Claims. (Cl. 338-276) ABSTRACT OF THE DISCLOSURE A resistorcomprising an elongate core bearing an elongate coating of resistivematerial thereon, and terminal means comprising a conductive tubularterminal member and a conductor having two end portions connected by abight, the bight extending around the core in contact with the resistivematerial and drawn tightly under tension against the resistive material,and the end portions of the conductor secured under tension in thetubular terminal member and electrically connected to the latter.

The invention herein disclosed pertains to terminal connections invariable resistors, and more particularly to improvements in terminalconnections for resistance elements of the composition type wherebyaccuracy of determining an electrical end of an element is improved andpermanency, reliability and conductivity of-the termination areimproved.

Various types of terminal structures have been devised with a viewtoward efiecting accurate end-points and low-resistance terminations forvariable resistors in the composition-element (molded, cermet, ormetal-film) class. A typical example is noted in US. patent toSchellenger, No. 2, 1=18,l12, in which special groovesare made andfilled with material offering good conductivity between the body of theresistive composition element and metallic terminals. Also the problemis discussed in US. Patent No. 2,736,783. While the same problem ispresented in the case of wire-wound potentiometer resistance elements(as indicated in U.S. Patent No. 2,783,343), the solution is there muchsimplified by the availability of an end boundary furnished by aselected turn of wire; whereas there is no such boundary available in anarcuate or helical composition-type resistance element. By the presentinvention there are provided means and method for producing a verydistinct and effective termination for that type of resistance elementin which an'insulated curved or helical wire-like core or mandrelcarries adherent thereto an elongate curved mass of resistivecomposition. The electrical end of the element, at which the resistanceexhibited between the potentiometer wiper or contact and the adjacentterminal is desirably zero, is very sharply defined and is such that asthe wiper or contact approaches it the exhibited resistance reaches zerovalue at the same mechanical position on every cycle of operation. Suchaccurate definition of the electrical ends of the elements is of greatimportance in precision potentiometers, such as those employed in servosystems.

The invention accomplishes the aforementioned results by providing adistinct sharply-defined fixed metal-t0- composition boundary which isdisposed at right angles to the path of the wiper, and a highlyconductive (substantially zero-resistance) path from the boundary to thepotentiometer terminal. The resistance element proper, of the characterof an applied resistive coating or layer extending along and firmlyadherent to an elongate insulative wire-like core or mandrel along oneside or longitudinally-extending zone of the mandrel, i-s transverselycrossed by a conductive member that preferably is in the form of a thinfiat ribbon of conductive metal such as nickel or silver alloy, theconductive member being drawn 3,356,981 Patented Dec. 5, 1967 'ice undertension tightly into contact with the resistive material and theremaintained under tension. Thus one edge of the conductive member definesa line-like boundary extending transversely across the resistivecoating, at which boundary the resistance of the electrical path to theterminal abruptly drops substantially to zero value. The preferably flatribbon-like conductive member is placed under tension, as by being drawnaround the resistance element :and drawn into contact with a tubularterminal and while drawn taut is fusion-united to the terminal, wherebya highly-conductive low-resistance connection of the element boundary tothe terminal is effected. To aid in insuring permanency of the contactor connection between the conductive member and the resistive material,conductive adhesive is preferably applied over at least a portion of theconductive member and adjacent resistive material adjacent the edge ofthe conductor distant from the noted boundary; and the tubular terminalis filled with molten metal, such as solder, which upon cooling servesto place the conductive member under high permanent tension. Initialfusion-uniting of end portions of the tensioned conductive ribbon to theterminal is conveniently effected by spot-Welding, whereby to facilitateapplication of solder as will be explained.

The preceding brief general description of the invention makes itevident that a principal object of the invention is to provideimprovements in variable resistor composition resistance-elementterminations.

Another object of the invention is to provide improved means and methodsfor electrically terminating composition-type resistance elements invariable resistors.

An additional object of the invention is to provide improvements indemarking the electrical boundary between an end of a compositionresistance element and a variable resistor terminal.

Other objects or advantages of the invention will hereinafter be statedor made evident in the appended claims and following description of anexemplary van'a'ble resistor incorporating a preferred embodiment oftermination according to the invention, the description containingtain'conventional parts having been removed and portions broken away forconvenience in illustration of pertinent details of the invention;

FIGURE 3 is a sectional view, to enlarged scale, of

structure depicted in part in FIGURES 1 and 2, taken as indicated bybroken line 33 in FIGURE 2, with a portion broken away; and

FIGURE 4 is a fragmentary sectional view, taken as indicated by brokenline 4--4 in FIGURE 2, to a different scale and with a portion brokenaway, showing relationships of parts according to the invention.

Referring first to FIGURE 1, the exemplary variable resistor, selectedfor convenience in this instance to be in the form of a multi-turnhelical-element precision potentiometer, is denoted generally by ordinal10. The resistor comprises a housing of any desired configuration, hereshown as a generally cylindrical shell or barrel 12 having end caps 14and 16 through the latter one of which the conventional rotary adjustingshaft 18 extends. Accessible inside the barrel of the housing forconnection to ends of a resistance element, and extending outwardlythrough apertures formed in the barrel, are first and second preferablytubular or hollow terminals 20 and 22, respectively. Similarly, a thirdterminal 24, which may be solid or like the others, is provided to whichelectrical connection from the potentiometer wiper contact is providedin manner and by means well known in the art.

Disposed against the inner wall surface of the barrel 12 and preferablyseated in a curved or helical groove formed in that wall, is aninsulated resistance-element core or mandrel 30 of helical form orconfiguration, as may be usual in helical-element precisionpotentiometers as depicted for example in US. Patents Nos. 2,815,422;2,850,- 606; 2,866,053; and 2,866,056. Preferably, but not necessarily,the core or mandrel is of circular or round crosssection. Carried by theinsulated mandrel and extending along an elongate surface zone or areaof the exposed inner surface thereof is a composition resistance elementproper 30c, preferably applied and formed and of characteristicsdisclosed in application for Letters Patent of the present inventor,Ser. No. 468,508, filed of even date herewith and entitled VariableResistor Element. The element proper, or resistive component of theresistor, is a hardened-insitu coating or strip of resistive compositionthat preferably is under some degree of compressive stress, as describedin the aforementioned application, to which reference may be had fordetails. The core or mandrel is, in usual manner, cut from a longhelical coil prior to installation in the housing, and is cutsufliciently long to provide the full extent of coating for wiping bythe potentiometer contact, plus extents suflicient to provide foreffective terminations or terminal connections, as will presently bemade evident. For example, a mandrel of 355 extent might be severed foran element for a single-turn rotary precision potentiometer, or anextent of 3630i10 might be severed for the helical element of a ten-turnrotary precision variable resistor, as will be evident to those skilledin the art.

Since the termination structures are or may be substantially identicalat opposite ends of the curved or helical element, only that at terminal20 of the exemplary ten-turn variable resistor will be described indetail. As indicated in FIGURES 2 and 3, terminal 20. is a conductive,shaped, preferably tubular or longitudinally slotted body having ashoulder or flange 20f fitted against a complementary surface providedon the exterior of insulative barrel 12, and having further an innerportion 20i firmly affixed in a hole formed through the barrel wall.

- The terminal is firmly afiixed to the barrel, as by a press fit aidedif desirable by adhesive, in a fashion known in the art. The exteriorend of the terminal is preferably provided with a head 20h as may beusual to facilitate formation of soldered connections to circuitconductors.

According to the invention, a very precise electrical end of theresistance element, and permanently stable and extremely low-resistanceend connection, are formed and provided by forming a loop or bight ofstrong highly conductive metal around an end portion of the mandrel andcomposition, and drawing tightly the bight and securing the same underpermanent tension. The loop or bight at least is preferably of flatstrip-like form, and for convenience the entire conductive memberforming the loop may be of flat strip form. Thus, as is indicated inFIGURES 2 and 3, a flat strip or ribbon-like conductor 22 extendsinwardly through the recess in the interior of terminal 20, forms a loopor bight 22b around the inner surface of an end portion of coating 30cand the mandrel 30, and extends outwardly through the terminal 20. Theexternal reaches of the conductor 22 are subjected to considerabletension and thus the bight or loop is brought into permanent and firmphysical and electrical contact with the resistive composition adjacentthe end of mandrel 30. After tensioning of the bight and adjacentreaches of conductor 22 the external reaches are drawn divergently apartand into contact with the end surface of terminal head 20h and are thereheld captive, preferably by being welded thereto, with the internalreaches and bight 22b still under tension. Thereafter, a portion of therecess in the interior of the terminal 20 is filled, or at leastpartially filled, with molten solder. Upon cooling (from the outsideinwardly) the solder 26 shrinks in an outward direction and appliesadditional tension to the conductor 22 in the upward direction as viewedin FIG- URE 3. Thus the terminal is at once sealed, the conductorpermanently tensioned with the bight tightly embracing the resistiveelement 30c, and the conductivity of the electrical path from theelement to terminal head 2012 increased substantially to bring theresistance of that path to an extremely low and negligible value.

Prior to completion of the resistance-element termination procedureshereinbefore described, the resistive coating forming element proper 30eis removed for at least a short distance back from the end of the core30 in all instances in which core 30 is an insulated conductor, withcare being exercised to leave intact the enamel or like insulation,whereby there is left a gap (denoted by G in FIGURE 2) between the endof the wire core and the end of the resistive coating. Further,following drawing-up or tensioning of conductor 22 about the end portionof the resistance element, a small amount of conductive adhesive may be,and preferably is, applied over that portion of the resistive coatingremaining exposed next-adjacent the end of the core, and over theadjacent part of the bight of conductor 22, as indicated at A in FIGURE2. As is now evident, the tightly-tensioned conductor 22 now defines atE (FIGURE 2) a definite electrical end of the resistance element proper,and also provides an end-resistance of substantially zero value for thevariable resistor or potentiometer in which the element and terminal areinstalled. Since the termination structure and mode are or may be thesame at the other end of the resistance element (at terminal 22), thesame characteristics are or may be evident at both ends of element 30e.

Referring to FIGURE 4, it will be noted that the conductor 22 tightlyengages the resistive coating or element 30c along the entire arcuatewidth or transverse section of the element, so there is no narrowingeffect at the electrical ends of the element, as is the case when only aportion of the element is brought into contact with the terminalconductor. Any such narrowing produces undesirable nonlinear electricalcharacteristics in prior-art potentiometers, which undesirable featureis prevented by the described structure.

The preceding description of an exemplary variable resistorincorporating a resistance-element termination and procedure thereforhas not had reference to certain other of variable resistor componentssuch as the actuating shaft, contact and contact-carrier or slider, andelectrical connections between the contact and third terminal 24, sincethose components and arrangements thereof are conventional andillustrated in the pertinent prior-art patents and per se form no partof the present invention. Further, the preceding description has madeevident the attainment of the several objects of the invention. In thelight of the disclosure changes and modifications within the true spiritand scope of the invention will occur to those skilled in the art; andhence it is not desired that the invention be restricted to the exactdetails of the exemplary illustrated structure embodying and utilizingthe invention other than as required by the appended claims.

I claim:

1. In a resistor, a resistance element comprising an elongate core andan elongate coating of resistive composition adhering to the core:

support means, supporting the said core and coating,

in a definite position;

a terminal member supported in spaced relation to said resistanceelement by said support means said terminal member having aconductor-receptive recess; and

a slender electric conductor extending around and in electrical contactwith a short extent of said coacting of resistive composition, saidconductor being tensioned and forming a tensioned bight or bendextending around said core and in said bight compressively engaging saidshort extent of said coating, and end portions of said conductor beingsecured in recess in said terminal member and conductivcly connectedwith said terminal member and said conductor being under tensionadjacent said bight, whereby said conductor is maintained in firmcompressive electrical contact with said coating and provides anextremely low-resistance electrical end connection to said resistivecoating and forms an electrical end for said element.

2. A structure according to claim 1, in which said terminal member istubular and in which said conductor extends inwardly through theterminal member and around said coating of resistive composition andoutwardly through the terminal member, under tension, and in which endportions of said conductor are welded to said terminal member.

3. A structure according to claim 2, in which said tubular terminalmember is sealed with solder adherent to said conductor and to theinterior of said terminal member whereby said conductor is permanentlytensioned and the resistance of the electrical path from said coating ofresistive composition to said terminal member is reduced to a negligiblevalue.

4. In a variable resistor having a cylindrical insulation housing withan aperture through a wall thereof and a helical resistance-element coredisposed substantially circumferentially around the interior wallsurface of said housing:

a hollow terminal member affixed in said aperture and having a passagepassing therethrough and communicating with the interior of saidhousing;

a resistive coating disposed along the interior surface of saidresistance-element core and forming an elongate resistive element; and

a thin flat highly-conductive strip having end portions fusion-united tosaid terminal member outside of said passage and having two reachesextending through said passage and a bight extending around and inelectrical contact with an end portion of said resistive coating toprovide a termination for the latter, said bight and reaches of saidstrip being held under applied tension.

5. A variable resistor according to claim 4, in which the said passageof said terminal is sealed with solder, whereby to add to the tension insaid bight of said strip and to reduce the resistance of the electricalpath from said coating to said terminal member to a negligible value.

6. In a variable resistor having an elongate curved resistance elementstructure comprising a core of round cross-section and having aninsulated surface and said structure further having an elongate coatingof resistive material thereon providing a resistance element proper forthe variable resistor:

a resistance-element termination device providing an electrical end forsaid resistance element proper, said termination device comprising anelongate electrical conductor drawn under tension with a bight over saidresistance element proper and in electrical contact with the latter,

and means including a rigid metal terminal having a recess therein, inwhich recess end portions of said conductor are fusion-united to saidmetal terminal under tension, whereby said bight is held under tensionsecurely embracing said resistance element proper transversely latter toprovide a definite electrical end for said resistance element proper andwhereby an electrical termination of negligible resistance is providedfor said resistance element structure.

7. A combination according to claim 6, in which said means includessupporting means holding said metal terminal spaced from said resistanceelement structure, and in which reaches of said conductor between saidbight and said terminal are maintained under tension by solderfusion-united with said conductor and with said terminal.

8. A variable resistor comprising:

first means, including a generally cylindrical housing with endclosures, said housing having at least one aperture through the wallthereof between said end closures;

second means, including a curved resistance-element core disposed aroundthe curved interior wall surface of said housing, and said second meansfurther including an elongate resistive coating disposed at least alonga portion of an interior exposed surface of said core to provide anelongate resistance element proper for said variable resistor;

third means, including a metal terminal member mounted in said aperture,extending in part to the exterior of said housing for facilitatingelectrical connection thereto; and

fourth means, including an elongate slender electrical conductor havingfirst and second ends and an intermediate bight, said bight beingdisposed under tension transversely across and in electrical contactwith a portion of said resistive coating, said conductor having reachesthereof between said bight and said ends held under tension andfusion-united to said terminal member.

9. A variable resistor according to claim 8, said conductor being of theform of a slender flat strip, and said terminal member being of tubularform and in which resistor said conductor is fusion-united to saidterminal member by solder disposed in the tubular portion of theterminal member.

10. A method of providing a stable very-low-resistance termination foran elongate curved composition resistance element proper, said methodincluding as steps the following:

disposing an extent of the resistive composition transversely of andfacing away from the adjacent end of an elongate recessed terminalprovided in fixed position extending away from the resistance elementproper;

providing and passing a bight of a bare highly conductive member aroundand transversely across said element proper and drawing end portionsthrough the recessed terminal member; drawing taut the bight and reachesof said conductor in the recess of said terminal member to force thebight of the conductor into compressive electrical contact with saidresistance element proper; and

fusion-uniting tensioned end portions of said conductor to said terminalmember to provide a very-low-resistance electrical path from saidelement proper to said terminal member.

11. A method according to claim 10, including in said fusion-unitingstep, the sub-steps of first spot-welding each end portion of saidconductor to the terminal while the conductor is tensioned, andthereafter filling the recess of the terminal with molten solder andcausing the solder to chill and solidify in progression toward theresistance element so as to increase the tension in the conductor.

References Cited UNITED STATES PATENTS 2,736,783 2/1956 Daily 338--3122,815,422 12/1957 Lock 338l4=6 FOREIGN PATENTS 894,196 12/1944 France.

LARAMIE E. ASKIN, Primary Examiner. E. GOLDBERG, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,356,981 December 5, 1967 Robert L. Jones It is hereby certified thaterror appears in the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected below.

Column 2, line 48, for "enlarge" read enlarged column 5, lines 6 and 7,for "secured in recess" read secured in said recess line 71, for"latter" read of the latter Signed and sealed this 7th day of January1969.

(SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer

1. IN A RESISTOR, A RESISTANCE ELEMENT COMPRISING AN ELONGATE CORE ANDAN ELONGATE COATING OF RESISTIVE COMPOSITION ADHERING TO THE CORE:SUPPORT MEANS, SUPPORTING THE SAID CORE AND COATING IN A DEFINITEPOSITION; A TERMINAL MEMBER SUPPORTED IN SPACED RELATION TO SAIDRESISTANCE ELEMENT BY SAID SUPPORT MEANS SAID TERMINAL MEMBER HAVING ACONDUCTOR-RECEPTIVE RECESS; AND A SLENDER ELECTRIC CONDUCTOR EXTENDINGAROUND AND IN ELECTRICAL CONTACT WITH A SHORT EXTENT OF SAID COACTING OFRESISTIVE COMPOSITION, SAID CONDUCTOR BEING TENSIONED AND FORMING ATENSIONED BIGHT OR BEND EXTENDING AROUND SAID CORE AND IN SAID BIGHTCOMPRESSIVELY ENGAGING SAID SHORT EXTENT OF SAID COATING, AND ENDPORTIONS OF SAID CONDUCTOR BEING SECURED IN RECESS IN SAID TERMINALMEMBER AND CONDUCTIVELY CONNECTED WITH SAID TERMINAL MEMBER AND SAIDCONDUCTOR BEING UNDER TENSION ADJACENT SAID BIGHT, WHEREBY SAIDCONDUCTOR IS MAINTAINED IN FIRM COMPRESSIVE ELECTRICAL CONTACT WITH SAIDCOATING AND PROVIDES AN EXTREMELY LOW-RESISTANCE ELECTRICAL ENDCONNECTION TO SAID RESISTIVE COATING AND FORMS AN ELECTRICAL END FORSAID ELEMENT.